1. Field of the Invention
The present invention concerns a semiconductor device and the manufacturing method therefor, and more particularly it pertains to pn junction semiconductor devices using Group II-VI compound semiconductors and the manufacturing method therefor.
2. Description of the Prior Art
Research and developments in respect of pn junction light-emitting diodes (LEDs) using various kinds of semiconductor materials are under way at present. The wavelengths of light produced by conventional LEDs range roughly from infrared region to green color in the visible region. Some of their examples are as follows.
LEDS made of GaAs having an energy band gap of about 1.43 eV emit light in the infrared region having a peak intensity at about 9100 .ANG.. GaP LEDs (not doped with nitrogen (N)) emit light of green color having a peak intensity at 5500 .ANG., while GaP LEDs which contain nitrogen emit yellow color light having its peak intensity at about 5800 .ANG.. Also, in mixed crystals formed of Ga.sub.1-x Al.sub.x As it is known that, by varying the component ratio x, LEDs emitting red color light having a peak intensity of about 6500 .ANG. are obtained.
The wavelength for light emission at peak intensity appears in a wavelength region longer than the absorption edge of the semiconductor. The wavelength .lambda. of the absorption edge, which is the shortest wavelength possible for emission, is expressed by: ##EQU1## wherein: h represents Planck's constant;
c represents velocity of light; and PA1 E.sub.g represents energy band gap of the semiconductor material expressed by eV.
The wavelengths of emitting lights of known LEDs extend up to about 5500 .ANG. (green color). No LEDs have previously been provided which emit light having a higher energy than that mentioned above, (i.e., a light of a shorter wavelength blue-green, blue and violet color), although there is a strong demand for such LED's.
As discussed above, in order to obtain an LED which emits light in the blue-green, blue and violet region, it is necessary to use semiconductor materials having an energy band gap broader than that of Group III-V compound semiconductors such as GaAs, GaP or Ga.sub.1-x Al.sub.x As. As such semiconductors, there are Group II-VI compound semiconductors such as ZnSe(E.sub.g .apprxeq.2.8 eV). These semiconductors have broad energy band gaps, and have therefore attracted the interest of researchers. Nevertheless, owing to various technical problems, they have not yet been put to practice. The Group II-VI compound semiconductors such as ZnSe stated above have a broad energy band gap, and, for some time, crystals have been produced either as a photo-conductive semiconductor or as an electroluminescent semiconductor. As is well known, however, there has been technical difficulty in achieving free control of the conductivity types of Group II-VI compound semiconductors. Table 1 shows the conductivity type and energy band gap of Group II-VI compound semiconductors which have been obtained in the past.
TABLE 1 ______________________________________ ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Conductivity n n p n n n p Energy band 3.6 eV 2.8 eV 2.2 eV 2.5 eV 1.74 eV 1.5 eV gap ______________________________________
The semiconductor materials listed in Table 1 invariably have a broad energy band gap, and will bring about a very effective result if LEDs are manufactured with them. As stated above, however, free control of the conductivity type of such compounds has not been practicable. For ZnS, CdS or ZnSe a n-type conductivity is easily obtained. However, even doping an acceptor impurity in an attempt to obtain a p type material results in the doped materials remaining an n type or in a p-type material having a very high resistivity. Even when the material happens to become p type, its control is so difficult that the formation of a pn junction necessary for an LED has not been possible.